The present invention relates to a fiber optic receiver module which converts optical signals supplied through optical fibers into electric signals.
Generally, an optical communication system has a fiber optic receiver module which converts input optical signals into electric signals. FIG. 1 shows a known fiber optic receiver module. This module comprises output terminals 11, metal housing 12 and a circuit board 13 fixed within the housing 12. The circuit board 13 has a conductive pattern connected to the terminals 11. The terminals 11 extend outwardly from the housing 12. A light-receiving circuit and a photodetector 18 are mounted on the circuit board 13. The circuit comprises an IC (integrated circuit) 14, resistors 15-1 to 15-3, capacitors 16-1 and 16-2, and other electronic elements. The photodetector 18 is fitted in a receptacle 17 which may be connected to an optical connector (not shown). The input terminals 10-1 of IC 14, the output terminals 10-2 thereof, the terminals 10-3 of the resistors 15-1, 15-2 and capacitors 16-1, 16-2 and the terminals 20 of the photodetector 18 are connected to the conductive patterns. To shield all the electronic elements from the external electromagnetic field, the housing 12 is put into a metal case 19. The housing 12 and receptacle 17 are secured to the case 19 by screws or other fastening means.
When photo signals of about 10.sup.-7 - 10.sup.-6 W are input to the module through the optical connector (not shown), they are converted by the photodetector 18 into electric signals of about 10.sup.-7 - 10.sup.-6 A. The electric signals are supplied to the IC 14. The IC 14 amplifies these signals and then shapes their waveforms, thus generating voltage signals of a few volts. The voltage signals are output through the output terminals 11.
The fiber optic receiver module shown in FIG. 1 is disadvantageous. A stray capacitance is inevitably produced between the input lines of the light-receiving circuit and the output lines thereof. More specifically, a capacitance C exists between, on the one hand, the lead terminals 20, input terminals 10-1, and the conductive pattern connecting terminals 10-1 to terminals 20, and on the other hand, the terminals 10-2, output terminals 11 and the conductive pattern connecting terminals 10-2 to output terminals 11. Due to this capacitance C, the receiver module may erroneously operate or oscillate.
More specifically, as the voltage signals from the terminals 11 changes by .DELTA.V for time .DELTA.t, current i (=C.multidot..DELTA.V/.DELTA.t) is induced in the lead terminals 20 of the photodetector 18, in the input terminals 10-1 of IC 14, and in the conductive pattern of the board 13, which connects the terminals 10-1 to the terminals 20. The capacitance C is about 0.01 PF or less. Hence, this current is about 10.sup.-6 A if change .DELTA.V is 3 V and time .DELTA.t is about 30 ns. The current i is nearly equal to the signal current (about 10.sup.-7 - 10.sup.-6 A) supplied from the output terminals 20 of the photodetector 18. Here arises the risk that the module erroneously operates or oscillates. The current i increases as the fiber optic receiver performs its function faster, thus shortening the time .DELTA.t. It follows that the fiber optic receiver module shown in FIG. 1 cannot convert the high frequency optical signal into a voltage signal.
Since the module is not sealed in an airtight fashion, the resistors 15-1, 15-2 and capacitors 16-1, 16-2 are exposed to moisture and may sooner or later fail to operate correctly. When the fiber optic receiver module was put to the pressure-cooker test at 121.degree. and 2 atms, its ability remarkably deteriorated.